Door lock sound and one door lock only work intermittently

This happens with the door handle sensor and the key fob, sometimes when locking the car there is no sound and all the doors lock except the rear left door. If I manually lock the rear left door before using the touch sensor or the fob then it will make the locking sound.

But then the left rear door will start to lock with the sensor or fob just fine for days at a time, it will work normally, and then it'll stop working again and the locking sound will stop and I'll have to manually lock that door again. Any ideas? What's the connection between the locking sound not happening and the door not locking?